1. Field of the Invention
The present invention relates to preparation of fluorinated core-shell particles with water and oil repellency, and more particularly, to preparation of emulsion-type coating materials with good water and oil repellency using only small amount of expensive fluorinated monomer, which comprise a shell wherein a large amount of hydrophobic fluorinated monomers are included and a core wherein no fluorinated monomers are included.
The emulsion technique is applied in many uses such as painting, adhesives, textiles, fine chemistry, electric/electronic, automotives and metal industries. In the emulsion technique, the surfactant polymer materials are generally dispersed in water as superfine polymer particles. These emulsion polymers usually contain organic solvents. However, recent environmental regulations started to employ more strict measures on volatile organic solvents to require transition to emulsion polymers not to contain organic solvents.
Hydrophobic emulsion materials are very useful because they provide various materials with superior properties, such as water repellency, oil repellency, anti-fouling property, lubrication property, non-sticking property and low surface tension. These hydrophobic polymer materials can be prepared from fluorinated monomer monomers with perfluorinated groups. The perfluorinated group [CF3(CF2)n(CH2)2xe2x80x94] contained in these fluorinated monomers shows an extreme hydorphobicity. That is, the critical surface tension is about 8 dynes/cm and the surface energy is the lowest in all existing compounds. Because this perfluorinated group has very low surface energy, it is oriented toward air at the air/material interface. Therefore, materials containing this perfluorinated group become hydrophobic. For this reason, a molecular structure which assures effective orientation of the perfluorinated group at the outermost air/material interface is very important in the development of hydrophobic functional polymer materials.
Because particle size of the conventional hydrophobic emulsion materials is relatively large as 2-3 xcexcm, they do not penetrate into substrates effectively and the surface orientation of the perfluorinated group is insufficient. And, use of a large amount (more than 50 wt %) of expensive fluorinated monomers to obtain hydrophobic increases the production cost. Also, organic solvents including acetone are used to enhance penetration to substrates and film forming property. However, as mentioned earlier, the environmental regulations about organic solvents require products not containing organic solvents.
One of the ways to reduce the amount of the fluorinated monomer and orient it at the outermost air/material interface is to make the emulsion particle in a core-shell structure. That is, the emulsion particle is given a core-shell structure, wherein the shell contains many perfluorinated groups and the core contains few or no perfluorinated groups. In this hydrophobic functional material with core-shell structure, the hydrophobic shell is oriented at the outermost air/material interface of the substrate surface and the core is oriented inside of the surface, so that superior hydorphobicity can be obtained by small fluorinated monomer content. Because of this, it becomes possible to improve efficiency of the hydrophobic functional material while reducing production cost.
2. Description of the Prior Art
It is known that preparation method and condition are very important in producing particles with a core-shell structure. The common methods are particle swelling and multi-stage semicontinuous emulsion polymerization. In the particle swelling method, particles with a variety of structures such as inverse core-shell structure are obtained in addition to the core-shell structure, depending on the thermodynamic properties of monomers and intermediates, and the polymerization condition kind like surfactant and concentration. In the multi-stage semicontinuous emulsion polymerization, the core latex is emulsion polymerized individually and swollen on particles to form shells for a long time. Then, the reaction is initiated or monomers are added continuously after adding an initiator. In both methods, the possibility of particle formation due to secondary nucleation cannot be excluded because the shell is formed by adding initiators.
Known techniques of preparing core-shell structure particles are as follows. U.S. Pat. No. 5,798,406 and Japanese Pat. No. 6,192,342 disclose preparation of core-shell particles wherein monomers containing perfluorinated groups are included in the shell. However, the surface quality of thus prepared particles are poorer than that of particles with perfluorinated groups only. That is, the contact angle to water is below 110xc2x0, which is smaller than that of perfluorinated group (about 120xc2x0). Also, in this case, the oil repellency is predicted to be very low. Preparation of particles containing fluorinated monomers in the shell through 3-stage emulsion polymerization is reported [Marion, et al., Macromolecules, 30, 123(1997)]. However, in this method, the fluorinated monomers are used more than 20 w % of particles and the final particle size reaches about 300 nm. Preparation of core-shell particles with superior water repellency through multi-stage emulsion polymerization is reported [Thomas, et al., Macromolecules, 33, 8828(2000)]. In this method, the monomers having perfluorinated groups are used less than 10 w % of total particles. However, the preparation process is rather complicated because it comprises many stages. In the cited literatures, an anionic surfactant alone or a mixture of an anionic surfactant and a non-ionic surfactant is used as an emulsifying agent. This is effective in monomer emulsification as well as particle suitability, but it may cause poor adhesion of particles to the substrate.
The inventors of the present invention made efforts to overcome the above-mentioned drawbacks of the conventional fluorinated hydrophobic emulsion coating materials. The present invention was completed by preparing coating particles with core-shell structure, wherein a large amount of hydrophobic fluorinated monomers are contained in the shell and no fluorinated monomers are contained in the core.
Accordingly, an object of this invention is to provide a method of preparing core-shell coating material with superior water and oil repellency using small amount of expensive fluorinated monomers, wherein a large amount of fluorinated monomers are contained in the shell.